Prehnite is an
inosilicate mineral of
calcium and
aluminium with the
chemical formula Ca
2Al(AlSi
3O
10)(OH)
2. Limited
Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the
orthorhombic crystal system, and most often forms as stalactitic,
botryoidal, reniform or globular aggregates. Prehnite is brittle with an uneven fracture and a vitreous to pearly luster. It has a hardness of 6.5 on the
Mohs scale and its
specific gravity is 2.80 to 2.95, while its color varies from light green to yellow, with some specimens also colorless. Prehnite is used as a gemstone, and was first described in 1788 for an occurrence in the
Karoo dolerites of
Cradock in the
Eastern Cape of South Africa. It was named after Colonel Hendrik Von Prehn, the commander of the military forces of the Dutch colony at the
Cape of Good Hope from 1768 to 1780. This prehnite crystal, measuring 4.0 cm × 3.5 cm × 2.0 cm (1.6 in × 1.4 in × 0.8 in), was found in
Southbury, Connecticut, in the United States.
Photograph credit: Ivar Leidus